Accompanying the progression of computer technology, computer systems are being provided with increasingly have powerful and robust functions. Presently, computer systems further provide multimedia service for entertainment besides processing paperwork. Corresponding to the powerful functions of computer systems, the speed and performance of CPUs are dramatically advanced, which results in increasing power consumption of CPU. Accordingly, the power management of CPU is an important problem for conventional computer systems, especially for portable computers, which are powered from batteries, for the great power consumption reduces the power supplying time of the batteries, which results in a correspondingly limited operational time of the portable computer.
In order to enhance power usage efficiency of computer systems, most power management systems are based on Advanced Power Management (APM) and Advanced Configuration and Power Interface (ACPI) in recent years, wherein the ACPI carrying on the power management through the operating system has better efficiency. The power management states defined by the ACPI include: Global state (G state), Sleep state (S state), Device state (D state) and CPU state (C state).
FIG. 1 is a schematic drawing showing CPU state of the conventional ACPI. As show in FIG. 1, the CPU state of the ACPI includes C0 state, C1 state, C2 state and C3 state. The CPU executes instructions normally at the C0 state, while the C1, C2, and C3 states are different levels of low power sleep states for the CPU. According to the usage of the CPU, the operating system drives the CPU to enter the different levels of sleep states for reducing power consumption.
With reference to the states described above, the C3 state, a non-snooping sleep state, is a state in which the CPU does not process any event. Therefore, when a peripheral device connected with a computer system sends an interrupt event or a bus master request to the computer system, while the CPU is at the C3 state, the CPU is driven to wake from the C3 state and return to the C0 state for processing the interrupt event or the bus master request. After that, the operating system will not drive the CPU return to the C3 state until the usage of the CPU satisfies the conditions for entering C3 state.
The peripheral device sends the bus master request for the accessing the system memory. However, the CPU has to wake from the C3 state and enter the C0 state to process the bus master request, and it takes the CPU a period of time to satisfy the conditions of entering C3 state and return to the C3 state. Thus, additional power is consumed during the time that the CPU is at C0 state waiting to return to the C3 state. As a result, additional time is lost before the CPU can reenter the most efficient power saving state. As described above, this results in excessive power consumption, and a corresponding reduction in the usage time of the portable computers.